This invention relates generally to blood oxygen sensors of the kind that are useful in cardiac pacemakers, determining oxygen content by measuring the blood's reflectivity, and, more particularly, to leakage compensation circuits and methods for use in such blood oxygen sensors.
The cardiac pacemaker is perhaps one of the best known electronic marvels of modern medicine, and the implantation of a pacemaker in a patient has become almost a routine operation. The pacemaker pulses the patient's heart continuously over an extended period of time, or in the case of demand pacemakers, monitors the heart's natural operation and provides stimulating pulses only when the heart skips a beat. Pacemakers allow patients with heart problems that otherwise would have been fatal or incapacitating to resume relatively normal lives.
It will be realized by those skilled in the art that the modern pacemaker is a highly complex device, capable of event sensing, two-way telemetry, and sensing and pacing in either or both of the atrium and the ventricle of the heart. Such pacemakers may be finely tuned by the physician subsequent to implant, and the parameters adjusted to provide optimum pacing performance.
Despite the sophistication of such pacemakers, they are a compromise due to a single major difference between the healthy heart and a paced heart--namely, the response to activity or exercise. Variations in the cardiac stroke volume and systemic vascular resistance occur in the cardiovascular system due to physiological stresses such as exercise, temperature changes, postural changes, emotion, hypoglycemia, Valsalva maneuvers, etc.
To maintain adequate perfusion pressure and cardiac output under these stresses, it is necessary to adjust heart rate. The healthy heart might beat at 60 or fewer beats per minute during repose or sleep, and at 120 or more beats per minute during strenuous exercise, for example. The heart paced by a pacemaker that is non-rate responsive will typically beat at a constant rate of approximately 70 beats per minute.
It will be appreciated that the constantly-paced heart will supply more blood than is needed during sleep, and might even prevent the patient from sleeping restfully. Even more seriously, patients paced at 70 beats per minute experience substantial difficulty in engaging in strenuous activity. Even a moderate level of activity such as walking will cause difficulty in some patients. It is apparent that a pacemaker whose rate varies in response to physiological need represents a highly desirable device that will enable patients requiring pacemakers to lead normal, active lives.
Physiologically-responsive cardiac pacing must optimize cardiac rate to the level of metabolic need in the absence of normal variable cardiac rate. The simplest solution to this problem is atrial tracking pacing, where the patient has a full or partial AV block and a dual chamber pacemaker pulses the ventricle in response to normal cardiac activity sensed in the atrium. However, this technique is not possible in many patients with sinus bradycardia or atrial fibrillation, and so rate-responsive pacing is necessary to mimic the normal variable cardiac rate.
A variety of physiologically-responsive pacing systems have been proposed, which utilize a variety of physiological parameters as the basis for varying cardiac rate. These parameters include blood temperature, various sensed timing signals from the heart, pressure measured within the heart, respiratory rate, nervous system activity, physical activity, and blood chemistry.
Systems responsive to various blood chemistry parameters such as blood oxygen saturation are particularly worthwhile and effective. U.S. patent application Ser. No. 07/403,208, filed Sep. 5, 1989 and entitled "Oxygen Content Pacemaker Sensor and Method," which is assigned to the same assignee as this invention, discloses one such blood oxygen saturation sensor. That application is hereby incorporated herein by reference. The disclosed sensor includes an optical detector for measuring the mixed venus oxygen saturation, typically in the right heart. A diminution in the mixed venus oxygen saturation is used to produce a higher paced cardiac rate. The speed of this system is comparable to the time constant of the body, thereby enhancing its effectiveness.
The blood oxygen sensor disclosed in the prior application includes a light-emitting diode, i.e., LED, positioned within the right heart and arranged such that any light it emits is directed at the blood within the right heart, which reflects the light to an adjacent phototransistor. The amount of light so reflected is related to the blood's oxygen content. The phototransistor is part of a circuit that is connected in parallel with the LED. When an electrical current pulse is supplied to the LED, the phototransistor circuit begins integrating the resulting phototransistor current. When the integrated voltage reaches a predetermined threshold, the phototransistor circuit latches and diverts the current pulse from the LED, to terminate its generation of light. The time delay from initiation of the current pulse to latching of the phototransistor circuit is inversely related to the blood's oxygen level.
The blood oxygen sensor described briefly above operates very effectively in providing an accurate, repeatable measure of blood oxygen content. Sometimes, however, inaccuracies can result from the presence of unspecified resistance values arranged in parallel with the LED and phototransistor circuit. This resistance, which can arise from the intrusion of fluid into the electrical lead and/or lead connector associated with the sensor, robs electrical current otherwise intended to drive the LED and thus prevents a proper initialization of the sensor prior to supplying the current pulse to the LED.
In this case, the initialization of the sensor may not be fully accomplished by the time the current pulse is supplied to the LED. This could cause the circuit to prematurely latch, giving no indication of blood oxygen content. In this case, a rate-responsive pacemaker depending on the sensor would be unable to increase in rate. Thus, it is apparent that a compensation scheme is highly desirable.
It should therefore be appreciated that there is a need for an improved blood oxygen sensor that operates effectively to measure blood oxygen content, even in conditions where an unspecified resistance is arranged in parallel with the sensor. The present invention fulfills this need.